1. Field of the Invention
The present invention relates to an X-ray image intensifier for converting an X-ray image into a visible image.
2. Description of the Related Art
Generally, X-ray image intensifiers are widely used in medical X-ray image pickup devices or X-ray industrial TV sets for industrial nondestructive tests.
An X-ray image intensifier of this type has a vacuum envelope. The vacuum envelope has an input window for receiving X-rays. An arcuated substrate is arranged in the vacuum envelope so as to oppose the input window. An input fluorescent screen and a photoelectric layer are stacked in the above mentioned order on a surface of the substrate, which is opposite to the input window side. An anode and an output fluorescent screen are arranged on the output side of the vacuum envelope. In addition, a converging electrode is arranged along an inner side wall of the vacuum envelope.
X-rays radiated from an X-ray tube pass through an object to be imaged, the input window, and the substrate, and then converted into light by the input fluorescent screen. The light is converted into electrons by the photoelectric layer. The electrons are accelerated and focused by an electron lens constituted by the focusing electrode and the anode. The electrons are converted into a visible image by the output fluorescent screen.
The visible image is picked up by a TV camera, a cinecamera, or a spot camera, and the resultant image is used for a medical diagnosis.
Of the fluorescent screens used in the X-ray image intensifiers, a fluorescent screen has been recently used, whose film thickness is greatly increased compared with conventional fluorescent screens.
X-rays to be absorbed by an input fluorescent screen having a thickness of T can be given as: EQU 1-exp(-.GAMMA.T)
where .psi. is an X-ray absorption coefficient. FIG. 1 shows a relation between the thickness of the input fluorescent screen and the absorption rate. Referring to FIG. 1, a material of the input fluorescent screen is cesium iodide (CsI), and an energy of X-rays is 60 KeV. The absorption index of X-rays is increased with an increase in film thickness, and hence X-rays can be efficiently used. As a result, an X-ray dose and can be reduced and image quality can be improved.
When output images are observed after X-rays are uniformly radiated onto the X-ray image intensifier, it is sometimes found in an output image that a central portion is bright, whereas luminance is decreased toward a peripheral portion of the image. This is because compared with the central portion of the image, the peripheral portion of the image is expanded by a socalled electron lens in the X-ray image intensifier. With such an output luminance distribution, the dynamic range upon imaging cannot be effectively utilized for the entire screen surface. That is, a possible application range of the output image cannot be widened.
A known method of maximally flattening an output luminance range is disclosed in, e.g., Japanese Patent Disclosure (Kokai) No. 53-102663, wherein the film thickness of an input fluorescent screen is gradually increased from its central portion toward its peripheral portion, According to this method, the input fluorescent screen emits light by absorbing a larger number of X-rays at the peripheral portion than at the central portion. Therefore, in the output side, the luminance of the peripheral portion is increased, and the output luminance distribution can become close to a flat one.
However, this method cannot be applied to the X-ray image intensifier using the above-described input fluorescent screen having a large film thickness.
The reasons the method disclosed by Kokai cannot be applied will be described below. First, for the purpose of understanding of the reasons, by using a model it is determined how much light emitted from the input fluorescent screen will reach the photoelectric layer when X-rays are uniformly incident onto the input fluorescent screen. FIG. 3 shows the model. The conversion amount of X-rays converted into light at small portion dt located at depth t in the input fluorescent screen having film thickness T is proportional to the light amount at position t. Since the distance from small portion dt to the photoelectric layer is T - t, if the absorption coefficient of light in the input fluorescent screen is set to be .beta., an amount of light component of the light converted by small portion dt and reaching the photoelectric layer, can be given as: EQU .alpha.e.sup.-.alpha.t .multidot.e.sup.-.beta.(T-t) dt
where .alpha. is an X-ray absorption coefficient. Therefore, the amount of light component of the light converted by the entire input fluorescent screen can be obtained by integrating the above formula as follows: ##EQU1## This definite integral is calculated as follows: EQU .alpha./(.beta.-.alpha.).times.exp(-.beta.T).times.{exp [(.beta.-.alpha.)T]-1}.
Accordingly, the value of this definite integral reaches its peak value at a given value of T. After input fluorescent screens having various film thicknesses were actually manufactured and tested, a peak value of the light amount was obtained at the photoelectric layer. FIG. 4 shows the test result. This data is obtained by measuring the luminance of an input fluorescent screen composed of CsI as a single-element film. In this case, an energy of X-rays is 60 KeV.
If the film thickness of the central portion of the input fluorescent screen is set to be the one exhibiting this peak value so as to effectively use the X-rays, the above method of correcting the output luminance distribution cannot be applied. More specifically, even if the film thickness of the peripheral portion of the input fluorescent screen is increased with respect to the central portion, luminance is decreased. As a result, the plotted output luminance distribution shows a step convex shape. In addition, if the film thickness is further increased, the resolution is degraded because of diffusion of light. That is, the film thickness corresponding to the peak value of emitted light is regarded as the maximum film thickness to be practically used. Therefore, it is necessary to solve the problem, i.e., that when an input fluorescent screen having such film thickness is realized, the output luminance distribution cannot be effectively corrected.
In addition, another problem will be described. If the film thickness is not uniform in one fluorescent screen, the X-ray absorption coefficient is changed depending on the quality of X-ray. For this reason, even if the output luminance distribution is made flat for a give X-ray quality, the output luminance distribution is not flat for other x-ray qualities.